API 526 provides provides orifice designations such as D, E, F..... etc. for relief valves with corresponding orifice cross-sectional areas. However, most manufacturers of relief valves provide the same API 526 designations with different areas (higher than as given in API 526) using a de-rated coefficient of discharge for the orifice. They call it the "actual orifice area" or "actual discharge area" which when de-rated with their own coefficient of discharge becomes equal to the API 526 orifice area.

In practice, if you are calculating the flow rate for a relief valve and specifically for the purpose of inlet / outlet line sizing and for fulfilling the inlet pressure drop criteria of 3% of set pressure and outlet line pressure drop of 10% of set pressure (conventional relief valve) then you must use the "actual orifice area" in the API 520 Part 1 formulas for flow rate for a given orifice as per the specific manufacturer.

Today's blog entry provides an excel table for the various reputed relief valve manufacturers across the globe whose "actual orifice area" is higher than the area as given in API 526 corresponding to the particular alphabetical orifice designation. This table should be useful if somebody selects a specific model of a given manufacturer such as "Farris" or "Leser". Not all manufacturers or models are covered but some more famous manufacturers and their common models are covered for the benefit of the members and readers of "Cheresources" and they can use it to determine the actual flow rates using formulas of API 520 Part 1.

It is important to note that this excel workbook does not include orifice areas as given in ASME Section VIII and covers only API 526.

Members and readers are welcome to provide their comments on the blog entry and the attached excel workbook.